bur (n.) Look up bur at Dictionary.com
"prickly seed vessel of some plants," c. 1300, burre, from a Scandinavian source (compare Danish borre, Swedish hard-borre, Old Norse burst "bristle"), from PIE *bhars- (see bristle (n.)). Transferred 1610s to "rough edge on metal," which might be the source of the sense "rough sound of the letter -r-" (see burr).